Hi, I was just wondering if there was speculation on how the reusable falcon stages could be used to deliver to a propellant depot, supposing such a market was established. (either their own depot or one from another company storing liquid hydrogen)

What brought this to mind was just that a propellant depot seems a good purpose for a reusable due to repetitive missions and focus on cost/kg over other requirements, plus someone's statement that the reason crew and second stage could not be combined is the lack of abort, which is not a factor for propellent cargo.

Would they design a modified second stage and omit the dragon? Use a disposable tank in place of the dragon? Keep full 3-stage reusability? It seems an obvious potential market but I haven't seen a statement on how they would do it.

Any suitable launch vehicle could in principle supply a depot. In the case of a re-usable F9, I think the main thing which will drive the design is what propellant is to be delivered. If they are delivering LOX and RP-1 to a depot, then it would possible for a re-usable second stage with extended tanks also to be the tanker.

If the cargo is H2/O2, then the hydrogen would have to be carried in a separate tank which might constitute part of an extended reusable second stage. Or the F9R might carry a tanker as a separate spacecraft as a payload under the fairing. Whether it would be worthwhile to attempt to recover the tanker is impossible to say. There are too many unknowns.

For H2/O2 I think the best technical solution is something like ACES. But since it doesn't look like SpaceX are going to develop their own hydrolox engine, they would have to come up with a different concept.

Hi, thanks. Basically the options I came up with:* An extended second stage, or* Some sort of disposable tank instead of the Dragon, relies on second stage to perform docking.* Keep full three-stage reusability, modify only the dragon.

I guess there is after all one advantage to the third example, it would sure help mature the crew carrying technology if you performed essentially the same mission an order of magnitude more times unmanned than manned.

Still, the lack of discussion of this alongside the huge discussion of the MCT BFR.. made me wonder.. is Elon Musk not a propellant depot fan? Any comments from him on the subject?

2.) The second stage is stretched until the fuel capacity equals the max liftoff mass.

Then you just transfer the remaining fuel to the depot and land the second stage for reuse.

In theory, yes. But bear in mind that this will be significantly different in size to the stock U/S- the CG and CP will be balanced differently. Whatever clever scheme they are planning to enable reuse of the U/S might not work so well with a stretched version.

In theory, yes. But bear in mind that this will be significantly different in size to the stock U/S- the CG and CP will be balanced differently. Whatever clever scheme they are planning to enable reuse of the U/S might not work so well with a stretched version.

Maybe, but the first stage is even much more stretched and they want to land that first. Using a stretched upper stage tank for payload and landing it would be so much more convenient, weight- and costefficient that it would be a bonus if it can be done.

In theory, yes. But bear in mind that this will be significantly different in size to the stock U/S- the CG and CP will be balanced differently. Whatever clever scheme they are planning to enable reuse of the U/S might not work so well with a stretched version.

Maybe, but the first stage is even much more stretched and they want to land that first. Using a stretched upper stage tank for payload and landing it would be so much more convenient, weight- and costefficient that it would be a bonus if it can be done.

I wasn't thinking about the actual landing, I was thinking about re-entry.The video showed the U/S entering nose-first. This might be possible, but it gets a lot harder the longer the stage gets. Basically, the problem is that stretching the tank is moving the CP forward- adding more drag area- whilst the engine and undercarriage remain at the back.In an ideal world, the stage would be passively stable in re-entry attitude, which would mean locating significant mass at the forward end to counteract the engine and undercarriage mass. This is probably not going to happen, so I think they will use dracos for active stability control. However, there is a limit to how much control authority these little thrusters will be able to effect. The longer the stage becomes, the harder it will be to control. At some point, it will become impossible, in practical terms.

I wasn't thinking about the actual landing, I was thinking about re-entry.The video showed the U/S entering nose-first. This might be possible, but it gets a lot harder the longer the stage gets. Basically, the problem is that stretching the tank is moving the CP forward- adding more drag area- whilst the engine and undercarriage remain at the back.In an ideal world, the stage would be passively stable in re-entry attitude, which would mean locating significant mass at the forward end to counteract the engine and undercarriage mass. This is probably not going to happen, so I think they will use dracos for active stability control. However, there is a limit to how much control authority these little thrusters will be able to effect. The longer the stage becomes, the harder it will be to control. At some point, it will become impossible, in practical terms.

You are right. And reentry may not even be the hardest part. Before landing the stage needs to flip over and point the engine in the direction of the flight. A short stage might be stable enough, but the longer it gets the more difficult will the flip be.

Edit: The solution to both problems may be making the upper stage wider, not longer. I know there is the transport problem, but only for the first flight. As we are talking about reusable that is not so bad. It would not need to be wider than the payload fairing, so drag during launch should also be no problem.

Reading this thread, it sounds like reentry of the second stage may be the most controversal element of SpaceX's reusability plan, so designing two different reusable 2nd stages would not be at all trivial?

I still like the idea of a reusable TSTO at some point but the same arguments could apply to a TSTO cargo-only service to the ISS. Would people agree it won't be considered for a while because SpaceX want to mature their crew-carrying technology so want the absolute minimum of variations?

Is this a good reason to argue that unless we are looking a long way ahead, any spaceX reusable depot architecture would in fact look a lot like their ISS cargo-only architecture (which also looks like their crew architecture)?

How long term is this propellant depot to remain in space? Just a quick day or so? Years? Hydrogen would be difficult to store, if it were attempted. With the methane and oxygen it would be much easier, but I think it would require some extra shielding from the Sun, beyond what the second stage currently has. Is simply rotating enough to get by for time frame desired?

How long term is this propellant depot to remain in space? Just a quick day or so? Years? Hydrogen would be difficult to store, if it were attempted. With the methane and oxygen it would be much easier, but I think it would require some extra shielding from the Sun, beyond what the second stage currently has. Is simply rotating enough to get by for time frame desired?

In the case of a refillable propellant depot it will remain in space for centuries.

Although there may be some minor hardware upgrading at the 100% level during that time.

How long term is this propellant depot to remain in space? Just a quick day or so? Years? Hydrogen would be difficult to store, if it were attempted. With the methane and oxygen it would be much easier, but I think it would require some extra shielding from the Sun, beyond what the second stage currently has. Is simply rotating enough to get by for time frame desired?

The good thing is that in the case of hydrogen it isn't SpaceX's problem. I think ULA proposed a demonstrator based on a centaur stage that would lose no more than (and probably less than) 20% in a year? (someone correct me if I got that wrong) That was just a demonstrator without any active cooling and in low earth orbit where depots will be less efficient.

I guess it is also possible that SpaceX might only want to sell oxygen to an oxygen/hydrogen depot. Perhaps the hydrogen would be delivered closer to the actual mission, along with the stage that was going to burn it for example. I really dont know.

Methane is a given fact (Musk said it) so the depot if any will be methane. LOX could be stored or not, you could lift the LOX you need and take only the methane from the depot and save anyway volume and weight at lift off: I think it would be simpler.

As for stage 2 I think it is silly to do such an effort to put it in orbit and then take it down and reuse it.

It would be usefull instead to assemble stage 2 tanks one with the other to form a "modular" depot keeping all the extra spare methane the stages have due to security margins or to compensate underweight payloads. Engines would be detached from the tanks, so avionics boxes, and stacked togheter, one engine "inside" the nozzle of the other. Sometimes a dedicated mission with a longer second stage (and no dragon) would carry methane to fill them and afterwards its tank would open and take inside engines and avionics boxes all stacked together.This tank/container would have a heat shield and be "sealed" against water. It would come down to earth with the help of parachutes and rocket to slow down and splash in the sea. Something would be needed to avoid this weighty tank from sinking

As for how to disassemble engines form second stages it could be something on each stage or, better, a modified dragon attached to the modular deposit with the necessary tooling to do it (and to keep the depot in orbit).

...And if you do that, why not keep the propellant in "extended cab" second stages, fuel from them into the BEO vehicle, and then return them afterwards?

Schedule issues. (But you ou can launch to depot anytime when you have a "free" slot.)You would have some or many 2nd stages unavailable for other works.You would have to launch ALL those tanker flights in time.You would have to have at least one tanker as backup,....

When you have the capability to launch often, why wouldn't you pre-launch the propellant in the month preceding the mission?

And if you do that, why not keep the propellant in "extended cab" second stages, fuel from them into the BEO vehicle, and then return them afterwards?

I think this is the most basic reason: If your mission involves N launches and the chance of failure is F, you want to avoid this meaning chance of the mission failing is multiplied (FN) because one launch lost means all your other launches were wasted.

This means it is nice to have the fuel up there well in advance with time to schedule another fuel flight or two if necessary. Someone else here could give you a good estimate for scheduling a replacement flight but I think it is quite a while. Also if your later launches are delayed you don't want to have wasted your earlier fuel flights.

With a high flight rate I think a depot would become more natural, with fuel not dedicated to any specific mission.

Methane is a given fact (Musk said it) so the depot if any will be methane. LOX could be stored or not, you could lift the LOX you need and take only the methane from the depot and save anyway volume and weight at lift off: I think it would be simpler.

A spaceX depot for BEO spaceX missions using methane powered rockets would no doubt store methane, assuming they use depots

However I doubt SpaceX would say no to launching fuel for say a NASA funded propellant market. In this case it is likely to be LH2/LOX. In many ways I think this is the most interesting example of depots anyway: when several companies service the same depot each with their own approaches to lowering cost/kg.